The present invention concerns the ability of computer users to increase the amount of high speed random access memory (RAM) used in a particular computer system. In particular, many computers are designed so that it is relatively easy to increase the amount of RAM which is installed in the computer. Typically, at least in "workstation" computers, it is common to provide a set of memory slots, into each of which can be inserted an additional memory module. Thus, if there are say, eight memory slots, each of which accepts a one-megabyte memory module, the computer can be set up with anywhere from one to eight megabytes of memory, depending on the user's needs and financial resources.
A new layer of complexity can be added to the above described "memory slot" scheme. In particular, several or all of the slots can be designed to accept memory modules of differing sizes. For instance, each memory slot can be designed to accept memory modules which contain either one megabyte of RAM or four megabytes of RAM.
There are at least two very good reasons for providing memory slots which can be used with memory modules of two or more sizes. First, it allows for a much broader range of memory sizes to be used with the same computer, without unduly increasing the number of memory module slots that have to be provided. For instance, a single computer can allow for a minimum memory configuration of eight megabytes and a maximum of 256 megabytes by providing just eight memory module slots, each of which can accept either 8 Megabyte or 32 Megabyte memory modules, instead of having to provide thirty-two memory slots, each of which can accept only 8 Megabyte memory modules.
Second, it allows the computer manufacturer to enable the use of higher density memory modules than those which are currently available. For instance, while memory modules using 1 Megabit dynamic RAM chips are commonly available at the time that this patent application is being filed (early in 1990), memory modules using 4 Megabyte dynamic RAM chips will soon be available and modules using 16 Megabyte RAM chips will be probably be available in a few years. Thus the trend toward increasing memory densities can be anticipated by computer manufacturers by providing memory module slots which will allow the use of higher density memory modules than those which are available at the time that the computer is designed or manufactured.
This brings us to the problem which is the subject of the present invention. In particular, how are physical addresses to be assigned to each of the memory modules in the computer if the sizes of the memory modules in the module slots are not predefined (i.e., the sizes are unknown before the computer is turned on)? In many computer systems, it is assumed that the portion of the computer's address space assigned to primary memory (random access memory) is contiguous--i.e., in these systems there are no unused addresses between the addresses assigned to the memory modules.
It should be noted that the memory which is being assigned to a contiguous range of address values in these systems is herein called "primary memory". Primary memory does not include image buffers, read only memory, and the like, which are typically assigned predefined address ranges that are not available to the memory modules used for primary memory.
There are several ways that prior art systems handle the problem of assigning addresses to memory modules. One way is to put on each memory module a set of switches which control how the memory in the module will be addressed by the system. For instance, many IBM PC compatible computers use this approach. Whenever a new memory module is installed in such a system, a person must figure out how to arrange the memory modules into a legal configuration. The person then sets the configuration switches on the memory module and installs the memory module in the proper place in the computer.
Another prior art method of dealing with this problem is to replace the person and the configuration switches with some digital logic. In some systems the allocation of memory arrays is done completely with hardware and in some systems it is partially accomplished with software. In either case, the computer determines the sizes of the memory modules in its various slots, and then assigns address ranges to each module so that the memory modules occupy a contiguous section of the computer's address space. This process is generally performed by the computer whenever the computer is turned on or restarted, and the computed address values are then stored in an address decoder or similar circuit in the computer's memory management system.
Thus, such prior art computer systems require programmable address decoders, and special routines for determining what address ranges are to be assigned to each memory module. Furthermore, due to the basic addressing requirements of many such computers, there are often limitations on the mixtures of memory modules which can be used, and which memory module slots those modules must be placed in. In other words, the user must find the right reference manual and must faithfully follow the instructions in that manual in order to be able to add memory to the computer.